Micro mechanical devices are small structures typically fabricated on a semiconductor wafer using techniques such as optical lithography, doping, metal sputtering, oxide deposition, and plasma etching which have been developed for the fabrication of integrated circuits.
Digital micromirror devices (DMDs), sometimes referred to as deformable micromirror devices, are a type of micromechanical device. Other types of micromechanical devices include accelerometers, pressure and flow sensors, gears and motors. While some micromechanical devices, such as pressure sensors, flow sensors, and DMDs have found commercial success, other types have not yet been commercially viable.
Digital micromirror devices are primarily used in optical display systems. In display systems, the DMD is a light modulator that uses digital image data to modulate a beam of light by selectively reflecting portions of the beam of light to a display screen. While analog modes of operation are possible—that is modes in which the mirror deflection is a function of the input data or bias voltage—DMDs typically operate in a digital bistable mode of operation in which the mirror is fully deflected at all times regardless of the image data applied to the mirror.
Micromirrors have evolved rapidly over the past ten to fifteen years. Early devices used a deformable reflective membrane which, when electrostatically attracted to an underlying address electrode, dimpled toward the address electrode. Schlieren optics illuminate the membrane and create an image from the light scattered by the dimpled portions of the membrane. Schlieren systems enabled the membrane devices to form images, but the images formed were very dim and had low contrast ratios, making them unsuitable for most image display applications.
Later micromirror devices used flaps or diving board-shaped cantilever beams of silicon or aluminum, coupled with dark-field optics to create images having improved contrast ratios. Flap and cantilever beam devices typically used a single metal layer to form the top reflective layer of the device. This single metal layer tended to deform over a large region, however, which scattered light impinging on the deformed portion. Torsion beam devices use a thin metal layer to form a torsion beam, which is referred to as a hinge, and a thicker metal layer to form a rigid member, or beam, typically having a mirror-like surface: concentrating the deformation on a relatively small portion of the DMD surface. The rigid mirror remains flat while the hinges deform, minimizing the amount of light scattered by the device and improving the contrast ratio of the device.
Recent micromirror configurations, called hidden-hinge designs, further improve the image contrast ratio by fabricating the mirror on a pedestal above the torsion beams. The elevated mirror covers the torsion beams, torsion beam supports, and a rigid yoke connecting the torsion beams and mirror support, further improving the contrast ratio of images produced by the device.
All micromirror-based projection displays use pulse-width modulation to control the amount of light that reaches each pixel of an image plane. Typical pulse width modulation schemes divide a frame period into binary bit periods. Each image data bit in the input data word controls the operation of the mirror during one bit period. Thus, if the bit is active, the mirror is turned on during the bit period and light from a light source is directed to the image plane during the bit period. If the image data bit is not active, the mirror is turned off during the bit period and light from the light source is directed away from the image plane during the bit period. The human eye, or other photoreceptor, integrates the energy directed to each pixel to create the perception of intermediate intensity levels. Typical binary pulse width modulation systems divide the larger bit periods into two or more bit-splits which are distributed throughout the frame period. Spreading the contribution of the large data bits throughout the frame period eliminates some of the artifacts created by the binary pulse width modulator schemes.
While not described above, the creation of full-color image requires either three DMD spatial light modulators simultaneously producing monochromatic images. The three primary monochromatic images are superimposed to create a single full-color image. Alternatively, a single DMD is used in combination with a color wheel or other sequential filter mechanism. The color wheel divides the white light beam into three primary color monochromatic light beams that are sequentially modulated to create single-color sub-images. The three primary color monochromatic images are integrated by the viewer to create the perception of a single full-color image.
Although binary pulse width modulation provides a convenient means to create intermediate intensity levels and utilizes binary data that is easily processed to improve the displayed images, binary pulse width modulation systems require a very large amount of memory and processing hardware. Thus, although DMD-based display systems are capable of creating virtually perfect images, the cost of such image quality drives the DMD-based projection system out of the reach of many consumers. What is needed is a method and system for creating high-quality images with display systems having much less processing power